Polymer-based microfabricated thermal ground plane

ABSTRACT

Embodiments described herein relate to the concept and designs of a polymer-based thermal ground plane. In accordance with one embodiment, a polymer is utilized as the material to fabricate the thermal ground plane. Other embodiments include am optimized wicking structure design utilizing two arrays of micropillars, use of lithography-based microfabrication of the TGP using copper/polymer processing, micro-posts, throttled releasing holes embedded in the micro-posts, atomic layer deposition (ALD) hydrophilic coating, throttled fluid charging structure and sealing method, defect-free ALD hermetic coating, and compliant structural design.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional of U.S. Provisional PatentApplication No. 62/069,564, filed Oct. 28, 2014, titled POLYMER-BASEDMICROFABRICATED THERMAL GROUND PLANE; and is incorporated into thisdisclosure by reference in their entireties.

This application is a Continuation-In Part of patent application Ser.No. 14/857,567, filed Sep. 17, 2015, titled MICROPILLAR-ENABLED THERMALGROUND PLANE; which claims priority to U.S. Provisional PatentApplication No. 62/069,564, filed Oct. 28, 2014, titled POLYMER-BASEDMICROFABRICATED THERMAL GROUND PLANE; and U.S. Provisional PatentApplication No. 62/051,761, filed Sep. 17, 2014, titledMICROPILLAR-ENABLED THERMAL GROUND PLANE, all of which are incorporatedinto this disclosure by reference in their entireties.

This application is a Continuation-In Part of patent application Ser.No. 14/853,833, filed Sep. 14, 2015, titled VACUUM-ENHANCED HEATSPREADER; which claims priority to U.S. Provisional Patent ApplicationNo. 62/069,564, filed Oct. 28, 2014, titled POLYMER-BASEDMICROFABRICATED THERMAL GROUND PLANE; U.S. Provisional PatentApplication No. 62/051,761, filed Sep. 17, 2014, titledMICROPILLAR-ENABLED THERMAL GROUND PLANE; and U.S. Provisional PatentApplication No. 62/050,519, filed Sep. 15, 2014, titled VACUUM-ENHANCEDHEAT SPREADER, all of which are incorporated into this disclosure byreference in their entireties.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with government support under Grant No.N66001-08-C-2006 awarded by DOD/DARPA. The government has certain rightsin the invention.

SUMMARY

Some embodiments of the invention relate to polymer-basedmicrofabricated thermal ground planes (TGP) and methods formanufacturing the same. In accordance with some embodiments, a polymeris utilized as the material to fabricate the thermal ground plane. Otherembodiments include an optimized wicking structure utilizing two arraysof micropillars, use of lithography-based microfabrication of the TGPusing copper/polymer processing, micro-posts, throttled releasing holesembedded in the micro-posts, atomic layer deposition (ALD) hydrophilicand hydrophobic coatings, throttled fluid charging structure and sealingmethod, defect-free ALD hermetic coating, and compliant structuraldesign.

Some embodiments of the invention include a method for forming a thermalground plane. The method can include the following depositing a firstpolymer layer on a substrate; depositing a first sacrificial layer in afirst pattern on the first polymer layer; depositing a second polymerlater on the first sacrificial layer; depositing a second sacrificiallayer on the second polymer layer, depositing a first masking layer onthe second polymer layer having a plurality of exposed regions thatexpose portions of the second polymer layer; etching portions of thesecond polymer layer that are exposed by the first masking layer;removing the first masking layer; depositing a sacrificial vapor corelayer in a pattern on the second sacrificial layer; depositing a thirdpolymer layer on the sacrificial vapor core layer; depositing a secondmasking layer on the third polymer layer having a plurality of exposedregions that expose portions of the third polymer layer; etchingportions of the third polymer layer that are exposed by the secondmasking layer; removing the second masking layer; removing portions ofthe sacrificial vapor core layer using the holes in the third polymerlayer; removing the substrate; and/or sealing the holes in the thirdpolymer layer.

In some embodiments, after removing portions of the sacrificial vaporcore layer the third polymer layer forms a plurality of pillars.

In some embodiments, the first pattern comprises a negative of aplurality of pillars.

In some embodiments, the second polymer layer comprises a mesh layer.

In some embodiments, the masking layer comprises a metal. In someembodiments, the masking layer comprises a plurality of exposed regions.

In some embodiments, the sacrificial vapor core layer may beelectroplated on the second polymer layer. In some embodiments, thevapor core layer may include a plurality of pillars.

In some embodiments, removing portions of the sacrificial vapor corelayer using the holes in the third polymer layer further comprisesintroducing an introducing an acid through the holes in the thirdpolymer layer.

In some embodiments, the sacrificial vapor core layer has a thickness ofless than 200 microns. In some embodiments, the second polymer layer hasa thickness of less than 50 microns. In some embodiments, the firstpolymer layer has a thickness of less than 50 microns.

In some embodiments, removing portions of the sacrificial vapor corelayer using the holes in the third polymer layer further comprisesremoving portions of the first sacrificial layer.

In some embodiments, depositing a third polymer layer on the sacrificialvapor core layer further comprises depositing the third polymer layerinto cavities or channels in the sacrificial vapor core layer.

Some embodiments include thermal ground plane. The thermal ground planemay include a bottom layer; a mesh layer bonded with the bottom layer avapor core having a plurality of pillars bonded with the mesh layer; anda top layer.

In some embodiments, either or both the top layer and the bottom layercomprise an in-plane wavy structure. In some embodiments, either or boththe top layer and the bottom layer comprise an out-of-plane wavystructure.

In some embodiments, either or both the top layer and the bottom layercomprise a polymer. In some embodiments, the plurality of pillarscomprise a polymer. In some embodiments, the mesh layer comprises apolymer.

In some embodiments, the mesh layer comprises one or more pillars and/orone or more microposts. In some embodiments, the one or more pillarshave a star cross-section shape.

These illustrative embodiments are mentioned not to limit or define thedisclosure, but to provide examples to aid understanding thereof.Additional embodiments are discussed in the Detailed Description, andfurther description is provided there. Advantages offered by one or moreof the various embodiments may be further understood by examining thisspecification or by practicing one or more embodiments presented.

BRIEF DESCRIPTION OF THE FIGURES

These and other features, aspects, and advantages of the presentdisclosure are better understood when the following Detailed Descriptionis read with reference to the accompanying drawings.

FIG. 1A illustrates the operational principle of a TGP according to someembodiments.

FIG. 1B illustrates a TGP with an evaporator, vapor core, condenser, andliquid wicking structure according to some embodiments.

FIG. 2A is a graph illustrating a range of possible micropost geometriesfor a given constraint to prevent dry-out, with capillary pressurecalculated according to the Young-Laplace equation and viscous pressuredrops calculated according to Poiseuille's Law.

FIG. 2B is a schematic of a micropost design for a wicking structure 120with two different array geometries in two different regions.

FIGS. 3A and 3B illustrate an example process for fabricating a TGPaccording to some embodiments.

FIG. 4 shows an example array of micro pillars.

FIG. 5A illustrates throttled releasing holes in a TGP after copperrelease, before a releasing hole is filled with epoxy.

FIG. 5B illustrates throttled releasing holes in a TGP after a releasinghole is filled with epoxy.

FIG. 6 illustrates an example structure and/or sealing method.

FIG. 7A illustrates an out of plane structure schematic according tosome embodiments.

FIG. 7B illustrates an out of plane compliant structure according tosome embodiments.

FIG. 7C illustrates an example process flow to fabricate an out-of-planewavy structure according to some embodiments.

FIG. 8 illustrates an example in plane structure.

FIG. 9 illustrates a structure fabricated using a PDMS mold.

FIG. 10 illustrates a TGP with a laminated perimeter according to someembodiments.

FIG. 11A is a schematic illustration of a pillar spacing problem.

FIG. 11B illustrates an optimization curve for optimized pillar spacingaccording to some embodiments.

FIGS. 12A and 12B illustrate an example process for fabricating a TGPaccording to some embodiments.

FIG. 13 illustrates an example thermal ground plane.

FIG. 14 is a graph illustrating optimal considerations for vapor corethickness.

FIG. 15 is a graph illustrating optimal considerations for vapor corethickness.

FIG. 16 illustrates a thermal ground plane integrated with a printedcircuit board according to some embodiments.

FIG. 17 illustrates a cross-sectional view of a TGP coupled with a PCBaccording to some embodiments.

FIGS. 18A and 18B illustrate an example process for fabricating a TGPintegrated with a PCB according to some embodiments.

FIG. 19A illustrates electrical vias fabricated through the polymerlayers in a TGP according to some embodiments.

FIG. 19B illustrates electrical vias in a TGP that can be connected toelectrical interconnects, vias or bonding pads on the PCB.

FIG. 20 illustrates several pre-fabricated flexible circuit boardslaminated into a single PCB with a TGP according to some embodiments

FIG. 21 illustrates a top view of a TGP according to some embodiments.

FIG. 22A shows a side view of TGP cut through Section A shown in FIG.21.

FIG. 22B shows a side view of TGP cut through Section B shown in FIG.21.

FIG. 22C shows an end view of TGP 700 cut through Section C shown inFIG. 21.

DETAILED DESCRIPTION

Thermal management can be an important element in micro systems in orderto remove heat away from sensitive electronic and photonic components.Among passive thermal management techniques, heat pipes usingphase-change heat transfer processes can have effective thermalconductivities an order of magnitude higher than any solid. Embodimentsdescribed herein may use polymer material to fabricate the entire TGPstructure. This may allow for ultra-thin and/or flexible device. Thepolymer may allow monolithic fabrication process to minimize theassembly and even ease the integration with the cooling target.

Some embodiments may have many features such as, for example, a wickingstructure design utilizing two or more arrays of micro-posts,microfabrication of the TGP using copper/polymer processing, micro-postssupporting the vapor core to minimize deformation under high pressure,throttled releasing holes embedded in the micro-posts, atomic layerdeposition (ALD) hydrophilic and/or hydrophobic coatings, throttledfluid charging structure, defect-free ALD hermetic coating, and/orcompliant structural design, etc.

In some embodiments, a thermal ground plane may be a passive thermalmanagement device that uses phase-change processes to achieve thermaltransport with low thermal resistance. FIGS. 1A and 1B shows an exampleof a polymer-based super-thin TGP 100 according to some embodiments.FIG. 1A illustrates the operational principle of TGP 100, showing heatflow with dotted lines, vapor flow in dashed lines, and liquid flow insolid lines.

The TGP 100 shown in FIG. 1B includes an evaporator 105, vapor core 115,condenser 110, and liquid wicking structure 120. In TGP 100, a heattransfer fluid (HTF) such as, for example, water and/or ammonia, in bothliquid and vapor phases may be disposed within an internal cavity of theTGP. Heat may be absorbed at one section (the evaporator 105), causingthe HTF in that area to evaporate. The heat may be carried by theevaporated HTF through the vapor core 115 to the cold section (thecondenser 110) where the HTF condenses and rejects the heat from thecondenser 110 to the outside. The now liquid-phase HTF is pulled back tothe evaporator 105 with capillary forces that result from the geometryand surface chemistry of the liquid wicking structure 120, resulting ina closed fluid circulation cycle which experiences liquid-vaporphase-change including evaporation and condensation. The liquid wickingstructure 120 may include pillars and/or a mesh (e.g., a woven metallicor polymer mesh).

In some embodiments, the wicking structure 120 may be fabricated as anarray of microposts. The design of such a micropost array can beoptimized in such a manner to minimize the thermal resistance betweenthe evaporator 105 and the condenser 110, while maintaining certaingeometric constraints such as a prescribed height, width, and/or lengthof the entire device, and/or operation parameters such as averagetemperature and wattage of heat to be transported. The optimizationprocess may include two steps: first an acceptable range of pillar arraygeometries may be determined that prevents the system from experiencingdry-out, then the geometry which minimizes thermal resistance may befound from within that range.

Dry-out may occur when liquid is not pumped back to the evaporator 105from the condenser 110 through the wick as fast as it is evaporated inthe evaporator 105. The liquid wicking structure 120 may provide enoughcapillary pumping pressure to overcome pressure losses associated withthe vapor flow through the vapor core 115 and liquid flow through thewicking structure 120. A denser array of pillars may, for example, leadto greater capillary pumping pressure but also higher liquid pressuredrop. In some embodiments, in order to lower the pressure dropassociated with the wicking structure 120, a second array of micropostswith a wider geometry may be fabricated between the evaporator 105 andthe condenser 110.

FIG. 2A is a graph illustrating a range of possible micropost geometriesfor a given constraint to prevent dry-out, with capillary pressurecalculated according to the Young-Laplace equation and viscous pressuredrops calculated according to Poiseuille's Law. In this example, theconstrained total height is 0.2 mm, the operating temperature is 325 K,and the dissipating heat is 5 W.

FIG. 2B is a schematic of a micropost design for a wicking structure 120with two different array geometries in two different regions. A firstregion may include either or both the evaporator region 105 and/or thecondenser region 110. A second region may include an axis region 135.For example, the cross-sectional area of the pillars in the first regionmay be greater than the cross-sectional area of the pillars in thesecond region. As another example, the space between adjacent pillars(e.g., the average space between adjacent pillars) in the first regionmay be different than the space between adjacent the pillars in thesecond region.

Once a range of geometries is found, the thermal resistance of allacceptable geometries may also be found. Some details about findingthermal resistances may include thermal conduction through the solidcasing material and/or vapor transport losses. Total thermal resistancesin an acceptable regime are shown in FIG. 2b for given constraintsaccording to some embodiments.

Some embodiments may include fabrication of a TGP. For example, someembodiments may include microfabrication of a TGP using copper/polymerprocessing that may allow the geometric features to be controlled bylithography. This may allow, for example, for control of the geometry inorder to attain any type of thermal performance.

FIGS. 3A and 3B illustrate a process 300 for fabricating a TGP accordingto some embodiments. Although illustrated as discrete blocks, variousblocks may be divided into additional blocks, combined into fewerblocks, or eliminated, depending on the desired implementation.

At block 1, a polymer 310 may be deposited onto a substrate 305 usingany polymer deposition technique known in the art, such as, for example,casting, painting, thermal spray, dipping, and/or spin-coating. Thesubstrate 305 may include a glass substrate. The polymer may include anytype of polymer material such as, for example, polyimide or any otherpolymer material.

At block 2, a first sacrificial layer 315 may be deposited on thepolymer 305. In some embodiments, the first sacrificial layer mayinclude a plurality of pillars. The plurality of pillars, for example,may have any number of sizes and/or patterns. The first sacrificialmetallic layer 315 may include a metal such as, for example, copper,aluminum, etc. In some embodiments, the first sacrificial layer 315 maybe deposited on the polymer layer 310 by first depositing a seed layerand then electroplating the sacrificial layer 315 to the desired height.In some embodiments, the first sacrificial layer 315 may be etched intoa negative having a pillar pattern using a photolithographic technique.In some embodiments, the etched away regions of the metal may becomepolymer pillars using a polymer processing technique including casting,painting, thermal spray, dipping, and spin-coating.

At block 3, a second polymer 320 may be deposited onto the sacrificiallayer 315 and/or the first polymer layer 310. The second polymer 320,for example, may include a pillar and mesh layer (e.g., a woven metallicor polymer mesh layer). The second polymer 320, for example, may fill atleast a portion of etched portions of the sacrificial layer 315 and/orcavities in the sacrificial layer 315.

At block 4, a masking layer 325, for example, comprising a metal such ascopper, may be deposited on the polymer 320. The shape, designed and/orpattern of the masking layer 325 may be defined by photolithographyand/or may be etched. For instance, the masking layer 325 may haveexposed regions 326.

At block 5, etching such as, for example, reactive ion etching (RIE) orany other etching technique may be used to remove portions of the secondpolymer layer 320 exposed by the exposed regions of the masking layer325. This etching may create the mesh structure 330 within the secondpolymer 320 layer. Following the etching, the masking layer 325 may beremoved.

At block 6, a sacrificial vapor core layer 335 may be deposited on thesecond polymer layer 320. The sacrificial vapor core layer 335 maycomprise a metal such as, for example, copper. In some embodiments, thesacrificial vapor core layer 335 may be electroplated on the secondpolymer layer 320. Support cavities and/or inverted pillars and/orsidewalls in the sacrificial vapor core layer 335, for example, may bedefined into the vapor core layer using any type of photolithographyand/or etching technique.

Turning to FIG. 3B, at block 7, a third polymer layer 340 may bedeposited onto and/or into cavities in the sacrificial vapor core layer335. The third polymer layer 340 may include pillars 341, side-walls,and/or a top layer 343. In some embodiments, the pillars may have acircular, polygonal, square, rectangular, or star shaped cross-section.

At block 8, a second masking layer may be deposited in a pattern to formrelease holes using a lithography technique. The second masking layermay be etched away along with the portions of the third polymer layer340 beneath the release holes creating release holes 345 through thepolymer layer.

At block 9 the sacrificial vapor core layer 335 may be removed with anacid that is introduced through the release holes 345 creating the vaporcore 350. The sacrificial layer 315 may be removed with the acid that isintroduced through the release holes 345 creating the liquid channel355.

At block 10, the device may be removed from the substrate 305 with asubstrate etchant (for example, hydrofluoric acid for a glasssubstrate).

At block 11, the releasing holes 345 may be filled with a polymer orepoxy to seal the top layer.

In some embodiments, the polymer layer 310 may have a thickness lessthan about 50 microns such as, for example, less than 25, 20, 15, 10,etc. microns.

In some embodiments, the third polymer layer 340 may have a thicknessless than about 50 microns such as, for example, less than 25, 20, 15,10, etc. microns.

In some embodiments, the vapor core 350 may have a thickness less thanabout 500 microns such as, for example, less than 250, 200, 150, 100,etc. microns.

In some embodiments, the liquid channel may have a thickness less thanabout 50 microns such as, for example, less than 25, 20, 15, 10, etc.microns.

Although process 300 is illustrated as discrete blocks, the variousblocks of FIG. 3 may be divided into additional blocks, combined intofewer blocks, or eliminated, depending on the desired implementation.

In some embodiments, materials other than copper can be used as thesacrificial layer(s) 315. These materials may include metals, as well asother polymers that can be subsequently released, for example thermallyor photolithically decomposable polymers such as polycarbonate ornorbornene-based polymers.

Various other manufacturing techniques may be used. For example, a sheetof rolled copper forming a copper base may be used for the vapor core.On this copper base, for example, the mesh structure 330 may be formed.The mesh structure 330, for example, may be electroplated throughlithographically-defined polymer electroplating masks. In someembodiments, the copper may grow into the shape desirable for theformation of a negative of the wicking structure. Once the wickingstructure is electroplated, the masks may be removed, and replaced by astructural polymer. The copper composing the vapor core can then bepatterned and etched to define the supporting pillars and sidewalls.This fabrication method may, for example, replace all or portions ofblocks 1-6.

In some embodiments, if the heat transfer fluid (HTF) experiencestemperatures above its normal boiling temperature, there may be apositive pressure difference between the vapor core of the TGP and theoutside ambient. In some embodiments, if the HTF ever experiencestemperatures below its normal boiling temperature, there will be anegative pressure difference between the vapor core of the TGP and theatmosphere around it. In some embodiments, pillars 360 may be embeddedinto the vapor core 350 to eliminate deformation of the top layer in thepresence of pressure differentials. The pillars 360, for example, may bedefined by etching the sacrificial copper that forms the vapor core. Anexample, of such micro pillars is shown in FIG. 4. In some embodiments,the pillars 360 may have a circular, oval, polygonal, square, starcross-sectional shape.

In some embodiments, the releasing holes 345 may be filled with sealingmaterial in order to seal the top polymer layer. In some embodiments,the sealing material may be introduced as a viscous fluid into thereleasing holes 345 that are suspended above the vapor core channel mayprovide some control over where the fluid will flow. Capillary forcesmay pull the fluid away from the releasing hole, preventing sealing andcausing flow resistance in the vapor core after the sealing fluidsolidifies. Furthermore, when the TGP is flexed, the polymer or epoxymay crack. In order to gain control of the flow of the sealing fluidbefore it solidifies; the releasing holes may be embedded in thepillars, with a throttled channel connecting the releasing hole to thesacrificial copper of the vapor core, as shown in FIGS. 5A and 5B. Whenpolymer or epoxy is introduced as sealing fluid into the releasing hole345, the throttled structure may prevent it from spreading into thevapor core 350. In some embodiments, when the TGP is flexed, the solidstructure of the pillar will lower the stress around the polymer orepoxy material and decrease the chances of cracks forming.

FIG. 5A illustrates throttled releasing holes in a TGP after copperrelease, before a releasing hole is filled with epoxy. FIG. 5Billustrates throttled releasing holes in a TGP after a releasing hole isfilled with epoxy. In some embodiments, epoxy is prevented from enteringthe vapor core by the throttled design.

In some embodiments, the mesh structure 330 may have a surface chemistrythat results in a low or high contact angle with the heat transferfluid. For example, a hydrophilic and/or a hydrophobic coating may beapplied to the mesh structure 330 to enhance evaporation orcondensation. Atomic layer deposition (ALD), for example, may be used toprovide a conformal layer of hydrophobic or hydrophilic coatings ontothe surfaces of the mesh structure 330. This deposition may take placebefore the release holes 345 are filled. Precursors to the ALD film candiffuse from a flow area above the TGP through the releasing holes, suchthat they deposit on the interior surface. In some embodiments, anadhesion layer of Al₂O₃ may be followed by a hydrophilic or hydrophobiclayer.

In some embodiments, the TGP may be evacuated with a vacuum, chargedwith a heat transfer fluid, and/or the charging region may besubsequently sealed. To facilitate this, a charging structure may befabricated. The charging structure may include a “T” junction. Thisstructure may be patterned into the vapor core layer, and may includemicroposts to prevent the channel from collapsing when the system isevacuated. One arm of the “T” connects to a macro-scale vacuum purge andfluid-charge system, while the other arm of the “T” connects to an epoxyinjection system. Once the system is evacuated of non-condensable gassesand charged with heat transfer fluid, epoxy may be injected into thecharging structure. A throttle at the end of the charging structure mayprevent the epoxy from entering the vapor core. In some embodiments,once the epoxy is cured, the charging structure may be cut away. Anexample structure and/or sealing method is/are shown in FIG. 6.

Polymers may not be hermetic, and they may allow diffusion of smallmolecules including air molecules through them. In some embodiments, theflow of gas molecules into the vapor core of the TGP introducesnon-condensable gasses, and the flow of HTF out of the TGP constitutes aleak—both of which can be stopped by applying an ALD inorganic barriercoating to the outer area and/or surface of the TGP. ALD films willfurther prevent leaks through the epoxy used to seal the fluid chargingstructures and releasing holes. In some embodiments, to provide adefect-free film, a polymer thin film may be used to encapsulate anymicron-sized particles or surface roughness associated with theprocessing or charging. Furthermore, after the ALD films are deposited,a polymer thin film may be used to encapsulate and protect the ALDfilms.

Although polymers can provide a naturally compliant material for theTGP, the ALD inorganic films may crack under moderate strain. In orderto prevent cracking or similar damage when the TGP is bent with a smallradius, in some embodiments, a compliant structure may be implemented.The compliant structure, for example, may reduce the strain associatedwith bending. In some embodiments, there may be two components to acompliant structure: an out-of-plane structure and an in-planestructure. In various embodiments, one or both may be used.

An example out-of-plane structure, which involves wavy structures in thetop-most polymer layer, is shown in FIG. 7A. The wavy structures canreduce the stiffness of the top-most layer, moving the neutral axiscloser to the bottom layer, therefore reducing the stress and strain atthe bottom when the TGP is bent, as shown in FIG. 7B. Such a structurecan be fabricated by forming a base micro-patterned mold (either byisotropic etching in the vapor-core sacrificial copper layer, orpatterning with other photo-definable polymers), coating the top polymerlayer, and pressing of the top polymer layer with a micro-patterned moldas the polymer cures, as shown in FIG. 7C.

FIG. 7A illustrates an out of plane structure schematic according tosome embodiments. FIG. 7B illustrates an out of plane compliantstructure reducing strain in the top and bottom layers to less than 1%.

FIG. 7C illustrates an example process flow to fabricate theout-of-plane wavy structures, including photographs of out-of-planepolymer wavy structures fabricated with this process. This process showsmethod to produce the structure using photo-definable SU-8 followed by apolydimethyl siloxane (PDMS) layer to round the corners of the SU-8 andprovide a barrier to prevent solvent incompatibility between the PDMSand the polyimide (PI).

An example in-plane structure is shown in FIG. 8. Multiple parallelchannels through which the liquid and vapor may flow are fabricatedwhere the TGP will experience flexing. These channels will be given awavy-profile in the plane of the polymer casting. In some embodiments,no additional processing may be needed to fabricate these structures.Both the gap between the parallel channels and the walls of the channelswill be minimized in size, in order to maximize the flow area and reducethe associated pressure drop. Wavy structures with large amplitude,short pitch, and thin channels may increase the compliance, while atincreasing the pressure drop. In some embodiments, an optimizationprocess may be used to select the geometry of the wavy structure thatmeets both the compliance criteria and minimize the associated pressuredrop.

In some embodiments, strain can be reduced if just one side of thelayers includes sinusoidal-wavy structures, which may have an amplitudethat is significantly thinner than the layer thickness and/or thickerthan the ALD film thickness. In some embodiments, the sinusoidal-wavymay have a wavelength no more than 5× the amplitude. For example, theoptimized wavy structure may have an amplitude of 2 μm and wavelength of10 μm.

In some embodiments, this structure can be fabricated using a PDMS moldthat is stamped into polyimide as it cures, as shown in FIG. 9.

In some embodiments, the fabrication of a TGP can be made withoutrelease-holes. For example, the pillars may be completely solid and/oretchant may be allowed to enter the TGP from the perimeter. In thiscase, the perimeter can be sealed by a flexible thermo-forming orthermo-setting polymer, such as fluorinated ethylene propylene (FEP),through a lamination process. FIG. 10 illustrates a TGP with a laminatedperimeter according to some embodiments.

In some embodiments, the size and/or spacing of the pillars may varybased on need. For example, a wide spacing can cause the top and bottomlayers to collapse, narrowing the hydraulic diameter and increasing theinternal pressure drop. For example, a narrow spacing may result in toomany pillars taking up the cross-sectional area that should be devotedto a flow path, resulting in a higher pressure drop.

In some embodiments, an optimum or nearly optimum pillar spacing can befound by minimizing the pressure drop through a given cross-sectionalgeometry as a function of pillar spacing, as shown in FIG. 11B. In thisexample geometry, the optimal spacing is 0.4 mm.

FIG. 11A is a schematic illustration of a pillar spacing problem andFIG. 11B illustrates an optimization curve for optimized pillar spacingaccording to some embodiments.

Some fabrication methods can cause leakage through the finalspin-coating layers. To avoid these leaks, in some embodiments, furthersealing of the structure may be facilitated by laminating a continuoussheet of thermosetting or thermoforming polymer covering the surface.

In some embodiments, condensation of the liquid along the top layer ofthe TGP may result in restrictions to the flow of vapor, and thusdegrading the performance. The liquid can be dispersed through a meshstructure along the top layer. This can be formed for example by etchingor electroplating an array micropillars into the sacrificial copperafter the pillar-structure has been defined. This can occur, forexample, after step 6 in FIG. 3.

Some embodiments may include an alternative to the fabrication schemeshown in FIG. 3. For example, the fabrication may be based around metalfoil as the sacrificial layer which eventually defines the largestlayer, e.g. the vapor core. Starting with the foil, the fabrication mayproceed as shown in FIG. 12A.

At block 1, a micropillar scaffold 1210 may be photo-patterned onto thebottom of a foil 1205.

At block 2, micropillars may be etched through the scaffold 1210 to formthe image of the wicking structure 1215.

At block 3, a polymer-coating 1220 may be deposited on the foil 12-5,which may form at least a portion of the wicking structure.

At block 4, an image of pillars 1225 may be photo-patterned and/oretched through the foil 1205 to form pillars 1225 in the foil 1205. Thepillars, for example, may be made from copper.

At block 5 in FIG. 12B, an additional micropillar scaffold may becreated as a top wicking structure 1230.

At block 6, a polymer-coating may be applied on top of the wickingstructure 1230, forming a secondary wick, pillars, and/or topmoststructural layer.

At block 7, the pillars 1225 may be etched away leaving a vapor chamber.

At block 8, an ALD coating may be applied and/or the perimeter may besealed. In some embodiments, a charge port 1245 may also be added forcharging the TGP and may be sealed once charged.

Although illustrated as discrete blocks, various blocks of FIGS. 12A and12B may be divided into additional blocks, combined into fewer blocks,or eliminated, depending on the desired implementation.

FIG. 13 illustrates a TGP 1300. The TGP 1300, for example, may befabricated using the process shown in FIG. 12. The TGP 1300, forexample, may have a thickness less than about 0.5 mm, 0.2 mm, 0.1 mm,0.05 mm, etc.

The TGP 1300 includes a top cover coat 1305 and/or a bottom cover coat1306. The top cover coat 1305 and/or the bottom cover coat 1306 maycomprise fluorinated ethylene propylene or other polymer material.

The TGP 1300 may also include a top polymer layer 1310 and/or a bottompolymer layer 1311. The top polymer layer 1310 and/or the bottom polymerlayer 1311 may comprise any type of polymer material such as, forexample, polyimide.

A bottom mesh 1320 may deposed on or may be coupled with the bottompolymer layer 1310. The bottom mesh 1320 may comprise a polymer layerthat may include a woven mesh or a plurality of pillars.

A top mesh 1325 may deposed on or may be coupled with the top polymerlayer 1311. The top mesh 1325 may comprise a polymer layer that mayinclude a woven mesh or a plurality of pillars. The top mesh 1325, forexample, may be a condensate collection mesh.

The vapor core 1330 may be formed between the top mesh 1325 and thebottom mesh 1320. The vapor core may include a plurality of pillars 1315that are disposed throughout the vapor core in any type of spacingbetween pillars 1315 and/or configurations. The bottom mesh 1320, forexample, may be a condensate collection mesh.

Edge seal 1335 may form a seal the vapor chamber 1330. The edge seal1335 may comprise fluorinated ethylene propylene or other polymermaterial.

In some embodiments, the TGP 1300 can be designed with a vapor-core 1330and/or liquid-channel thickness designed by minimizing the thermalresistance through the TGP 1300 while adhering to the physicalconstraints that the TGP 1300 be less than 0.1 mm in total thickness andthat the pressure drops associated with the liquid and vapor are smallerthan the capillary pumping pressure.

This can be done, for example, as shown in FIG. 14 and/or FIG. 15,showing an example or optimal vapor core thickness of 0.065 mm for adevice with a total thickness of 0.1 mm.

In some embodiments, a thermal ground plane 1600 can be integrated witha printed circuit board (PCB) 1605 as shown in FIG. 16. The TGP 1600includes liquid channels 1610, wicking mesh 1615, and/or vapor corepillars 1620. While two wicking mesh and liquid channels 1610 are shown,in some embodiments only one may be used. In some embodiments, heat maybe transferred from one side of the TGP 1600 to the other side of TGP1600. The liquid condensed on the bottom of the TGP 1600 may becollected and delivered back to the top of the TGP 1600. In someembodiments, one or two sets of wicking layers 1615 and/or liquidreturning channels 1610 may be used. In some embodiments, electricalvias 1625 may pass through the TGP 1600. For example, vias 1625 may passthrough the TGP 1600 through the vapor core pillars 1620. In someembodiments, heat generated in different components on the PCB 1605 maybe transferred to the TGP 1600 through thermal vias 1630.

FIG. 17 illustrates a cross-sectional view of a TGP coupled with a PCBaccording to some embodiments. Thermal vias and electrical vias andtheir connections to the TGP or through the TGP are illustrated in thefigure. The double sets of wicking layers and liquid returning channelsare also illustrated.

FIGS. 18A and 18B illustrate an example process for fabricating a TGPintegrated with a PCB according to some embodiments. A PCB 1800 isprovided in block (a). Thermal vias 1805 may be fabricated in the PCB1800 as shown in block (b).

At block (c), various TGP layers may be fabricated on top of the PCB1800. These TGP layers may include any TGP layer or structure describedin this document. These layers may include, for example, a bottompolymer layer 1815; a bottom sacrificial layer 1820; and a bottomwicking polymer layer 1825. The bottom sacrificial layer 1820 may be acopper layer.

At block (d) the wicking polymer layer 1825 may be etched with amicro-structured pattern 1830.

At block (e) pillars 1835 may be fabricated. The pillars 1835 maycomprise a polymer. The pillars 1835, for example, may be surrounded bya thick sacrificial copper layer 1840. When the sacrificial copper layer1840 is removed, the resulting space may form the vapor core of the TGP.

At block (f) a top polymer wicking layer 1850 and/or a liquid returningchannel 1845 layer may be fabricated.

At block (g) in FIG. 18B the fabrication of the multilayered structuresmay be completed with the top enclosure layer 1855 with release holes1860 etched. These release holes 1860 may be needed to introduce liquidor vapor etchant to remove the sacrificial copper layer 1840 and/orother layers.

At block (h) the sacrificial copper layer 1840 and/or other layers maybe removed by etchant introduced through the release holes 1860. Thevapor core 1865 may be created with the removal of the sacrificialcopper layer 1840. Other wicking and/or mesh layers may also be formedwith the removal of the sacrificial copper layer 1840.

At block (i) various portions within the TGP structure may be coatedwith atomic layer deposition layers for hydrophilic or hydrophobicproperties and/or for sealing polymer from water to be charged. Hermeticsealing of polymer may be important in some embodiments becauseoutgassing from the polymer could generate non-condensable gas thatwould degrade TGP performance. In addition, the release holes will besealed by another polymer, glass or metal materials.

At block (j) the air inside the structure may be sealed and/or may bevacuum-removed, after which water may be charged into the structure. Insome embodiments, the charging port can be a copper tube or polymerconnectors to be hermetically sealed by additional atomic layerdeposition.

At block (k) the charging port may be sealed after charging.

At block (i) an ALD coating may be applied to encapsulate the entireexterior surface of the TGP to hermetically seal the entire system.Various other sealing techniques may be used before or after charging ofthe TGP. It should be noted that all the ALD coatings mentioned hereincan be replaced by other coating processes such as chemical vapordeposition or physical metal deposition processes.

In some embodiments a single set of wicking layer and/or liquidreturning channel layers may be applied for a thinner TGP withevaporators and condensers placed on the same side of the TGP.

FIG. 19A illustrates electrical vias fabricated through the polymerlayers in a TGP according to some embodiments. Such a via fabricationprocess can include hole drilling process followed by electroplatingused by PCB manufacturing. FIG. 19B illustrates electrical vias in a TGPthat can be connected to electrical interconnects, vias or bonding padson the PCB.

In some embodiments, several pre-fabricated flexible circuit boards canbe laminated into a single PCB with TGP integrated as illustrated inFIG. 20. The top and the bottom flexible circuit boards can carryelectronic components. The flexible circuit boards can be fabricatedwith different structures for wicking, vapor core and liquid returningchannels.

Some embodiments of the invention may use a mixture of liquids as theheat transfer fluid. For example, some embodiments, may use somecombination of water, acetone, isopropanol, ethanol, methanol, etc., asthe heat transfer fluid. Some embodiments may use other natural andartificial fluids as the components of the heat transfer fluid. Someembodiments may use azetropic fluid mixtures whereby the vapor andliquid compositions are the same. Some embodiments may use non-azetropicmixtures, whereby the vapor-phase composition differs from theliquid-phase composition.

Some embodiments of the invention may use a fluid composed of a highsurface tension liquid (e.g. water, ammonia, etc., or mixtures thereof)and a high vapor pressure fluid (e.g. acetone, isopropanol, ethanol,etc. and mixtures thereof).

FIG. 21 illustrates a top view of another TGP 700 and FIGS. 22A, 22B,and 22C illustrate side views of the TGP 700 according to someembodiments. The TGP 700 includes a top layer (805 in FIG. 22) and abottom layer 705. FIG. 21 shows the TGP 700 with the top layer 805removed. The top layer 805 and/or the bottom layer 705 may includecopper and/or polyimide material. In some embodiments, the top layer 805and/or the bottom layer 705 may include layers of both copper andpolyimide.

The TGP 700 includes a mesh return layer 715 that includes a pluralityof return arteries 710 formed or cut within the mesh return layer 715.In some embodiments, the return arteries 710 may not extend into theevaporator region 720 of the TGP 700. In some embodiments, the width ofthe return arteries 710 may be less than 30 microns. In someembodiments, the width of the return arteries 710 may be less than 100microns. In some embodiments, the mesh return layer 715 may include awick. In some embodiments, the mesh return layer 715 may include a steelmesh such as, for example, a mesh with wires that are less than 50microns or 25 microns thick. In some embodiments, the mesh return layer715 may include a mesh with a simple weave at 200, 300, 400, 500, 600,700, etc. wires/inch. In some embodiments, the mesh return layer 715 maybe electroplated with copper. The mesh return layer 715 may include aspecified evaporator region that may provide a specific location for aheat source. In some embodiments, the mesh return layer 715 may have athickness of about 40 microns.

The mesh return layer 715 may have any number of shapes and/orconfigurations. In some embodiments, the mesh return layer 715 may havea polygonal or circular shape. In some embodiments, the mesh returnlayer 715 may have multiple sections without return arteries 710. Insome embodiments, the mesh return layer 715 may include any number,shape or configuration of return arteries 710. In some embodiments, thereturn arteries 710 may have one or more pillars or other mechanismsdisposed within the return arteries 710.

In some embodiments, return pillars 835 (see FIG. 22) may extend fromthe bottom layer 705 through the return arteries 710 of the mesh returnlayer 715. In some embodiments, the return pillars 835 may form vaporregions running parallel to the mesh return layer 715. In someembodiments, the return pillars 835 may form arteries in the adiabaticand/or condenser regions of the TGP 700.

The return pillars 835 may have at least one dimension that is smallerthan the width of the return arteries 710. In some embodiments, thesepillars may have at least one dimension (e.g., height, width, length,diameter, etc.) that is less than 10 microns. In some embodiments, thesepillars may have at least one dimension (e.g., height, width, length,diameter, etc.) that is less than 50 microns. In some embodiments, thesepillars may have at least one dimension (e.g., height, width, length,diameter, etc.) that is less 100 microns.

FIG. 22A shows a side view of TGP 700 cut through Section A shown inFIG. 21. In FIG. 22A, the TGP 700 is cut through a region where a returnarteries 710 extends along a portion of the mesh return layer 715. Asshown in FIG. 22A, the mesh return layer 715 is present in theevaporator region. The return pillars 835 are shown extending throughthe return arteries 710. The TGP 700 also includes a plurality of toppillars 825 disposed on the top layer 805. The top pillars 825 may haveat least one dimension (e.g., height, width, length, diameter, etc.)that is larger than the return pillars 835. The top pillars 825 may haveat least one dimension (e.g., height, width, length, diameter, etc.)that is larger than 0.25 mm, 0.5 mm, 0.75 mm, 1.0 mm, 1.25 mm, etc.

In some embodiments, the TGP 700 may include a micro wick layer 815. Themicro wick layer 815, for example, may include a plurality of pillars(e.g., electroplated pillars). The micro wick layer 815 may have atleast one dimension (e.g., pillar height, width, length, diameter,pitch, etc.) that is smaller than the return pillars 835. The micro wicklayer 815 may have at least one dimension (e.g., height, width, length,diameter, etc.) that is smaller than 5 μm, 10 μm, 15 μm, 20 μm, 25 μm,etc. The micro wick layer 815 may be aligned with the return arteries710.

FIG. 22B shows a side view of TGP 700 cut through Section B shown inFIG. 21. In FIG. 22B, the TGP is cut through a region without the returnarteries 710 extending along a portion of the mesh return layer 715.Instead, the mesh return layer 715 extends along the length of the TGP700 along this section of the TGP 700.

FIG. 22C shows an end view of TGP 700 cut through Section C shown inFIG. 21. In FIG. 22C, the TGP is cut through the mesh return layer 715showing both the mesh return layer 715 and the return arteries 710formed in the mesh return layer 715. In some embodiments, the returnpillars 835 may extend through the return arteries 710. In someembodiments, one or more of the return pillars 835 may contact one ormore of the top pillars 825.

In some embodiments, the top layer 805 and the bottom layer 705 aresealed along at least one edge of the top layer 805 and along at leastone edge of the bottom layer 705. In some embodiments, the top layer 805and the bottom layer 705 are sealed along at least two edges of the toplayer 805 and along at least two edges of the bottom layer 705.

In some embodiments, a buffer region can be created by design to collectand store any non-condensable gas through passive convection. Forexample, a space of a few millimeters can be formed in the area outsidethe mesh (e.g., outside the mesh region shown in FIG. 6). This space canbe added prior to bonding. This space may collect any non-condensablegases that would move to this space because of its different density,and thus its effect on evaporation and condensation can be reducedsubstantially.

Various other sealing techniques may be used such as, for example,thermosonic or thermo-compression bonding, ultrasonic welding, laserwelding, electron beam welding, electroplating; solder sealing withalloys with negligible reaction with water; and polymer bondingencapsulated by moisture barrier coatings such as atomic layerdeposition (ALD)-based coatings.

Some embodiments may include a pillar-enabled TGP. In some embodiments,the TGP may include a copper-cladded Kapton film that includes threelayers. These layers may, for example, include copper and Kapton layers.Each layer may be about 12 um thick. In some embodiments, a stainlesssteel woven mesh may be included and may have a thickness less than 75um. In some embodiments, the pillars may allow for fluid and/or vaportransport between the pillars under different mechanical loadings.

In some embodiments, a plurality of pillars may be formed on a copperlayer (e.g., the top layer and/or the bottom layer) using any of variouslithography lithographic patterning processes.

In some embodiments, a copper-encapsulated stainless steel mesh may besandwiched between the top layer and the bottom layer. The stainlesssteel mesh, for example, may have a weave that is less than 75 micronsin thickness. In some embodiments, the mesh may be copper encapsulated.In some embodiments, the mesh may be hydrophilic. In some embodiments,the reaction of the mesh with water may be negligible.

In some embodiments, a TGP may include a mesh-pillar wicking structure.The mesh-pillar wicking structure may allow the TGP to achieve a lowcapillary radius (high p umping pressure) in the evaporation regionsand/or a higher flow hydraulic radius (low flow pressure drop) in thefluid channel.

In some embodiments, a TGP may include pillars with rounded heads. Forexample, the pillars may be formed with controlled over-plating. In someembodiments, the pillars may form very sharp angle at the interfacebetween a pillar and the mesh bonded. In some embodiments, these sharpangles may be used, for example, to enhance the capillary p umping forcepulling the liquid returned from the condenser to the evaporator.

In some embodiments, a plurality of star-shaped pillars may beconstructed on either the top layer and/or the bottom layer that have astar-shaped polygon various cross section.

In some embodiments, a plurality of hydrophilic pillars may beconstructed on either the top layer and/or the bottom layer.

In some embodiments, heat rejection through condensation can bedistributed throughout the external surface of the TGP.

In some embodiments, pillars and/or spacers may be disposed on a layerwith densities (spacing between pillars or spacers) that vary across thelayer, with diameters that vary across the layer, with spacing that varyacross the layer, etc.

The Figures are not drawn to scale.

The term “substantially” means within 5% or 10% of the value referred toor within manufacturing tolerances.

Numerous specific details are set forth herein to provide a thoroughunderstanding of the claimed subject matter. However, those skilled inthe art will understand that the claimed subject matter may be practicedwithout these specific details. In other instances, methods,apparatuses, or systems that would be known by one of ordinary skillhave not been described in detail so as not to obscure claimed subjectmatter.

Some portions are presented in terms of algorithms or symbolicrepresentations of operations on data bits or binary digital signalsstored within a computing system memory, such as a computer memory.These algorithmic descriptions or representations are examples oftechniques used by those of ordinary skill in the data processing art toconvey the substance of their work to others skilled in the art. Analgorithm is a self-consistent sequence of operations or similarprocessing leading to a desired result. In this context, operations orprocessing involves physical manipulation of physical quantities.Typically, although not necessarily, such quantities may take the formof electrical or magnetic signals capable of being stored, transferred,combined, compared, or otherwise manipulated. It has proven convenientat times, principally for reasons of common usage, to refer to suchsignals as bits, data, values, elements, symbols, characters, terms,numbers, numerals, or the like. It should be understood, however, thatall of these and similar terms are to be associated with appropriatephysical quantities and are merely convenient labels. Unlessspecifically stated otherwise, it is appreciated that throughout thisspecification discussions utilizing terms such as “processing,”“computing,” “calculating,” “determining,” and “identifying” or the likerefer to actions or processes of a computing device, such as one or morecomputers or a similar electronic computing device or devices, thatmanipulate or transform data represented as physical, electronic, ormagnetic quantities within memories, registers, or other informationstorage devices, transmission devices, or display devices of thecomputing platform.

The system or systems discussed herein are not limited to any particularhardware architecture or configuration. A computing device can includeany suitable arrangement of components that provides a resultconditioned on one or more inputs. Suitable computing devices includemultipurpose microprocessor-based computer systems accessing storedsoftware that programs or configures the computing system from ageneral-purpose computing apparatus to a specialized computing apparatusimplementing one or more embodiments of the present subject matter. Anysuitable programming, scripting, or other type of language orcombinations of languages may be used to implement the teachingscontained herein in software to be used in programming or configuring acomputing device.

Embodiments of the methods disclosed herein may be performed in theoperation of such computing devices. The order of the blocks presentedin the examples above can be varied—for example, blocks can bere-ordered, combined, and/or broken into sub-blocks. Certain blocks orprocesses can be performed in parallel.

The use of “adapted to” or “configured to” herein is meant as open andinclusive language that does not foreclose devices adapted to orconfigured to perform additional tasks or steps. Additionally, the useof “based on” is meant to be open and inclusive, in that a process,step, calculation, or other action “based on” one or more recitedconditions or values may, in practice, be based on additional conditionsor values beyond those recited. Headings, lists, and numbering includedherein are for ease of explanation only and are not meant to belimiting.

While the present subject matter has been described in detail withrespect to specific embodiments thereof, it will be appreciated thatthose skilled in the art, upon attaining an understanding of theforegoing, may readily produce alterations to, variations of, andequivalents to such embodiments. Accordingly, it should be understoodthat the present disclosure has been presented for-purposes of examplerather than limitation, and does not preclude inclusion of suchmodifications, variations, and/or additions to the present subjectmatter as would be readily apparent to one of ordinary skill in the art.

That which is claimed:
 1. A method for forming a thermal ground planecomprising: depositing a first sacrificial layer in a first pattern on afirst polymer layer; depositing a second polymer layer on the firstsacrificial layer; depositing a second sacrificial layer on the secondpolymer layer; depositing a first masking layer on the second polymerlayer having a plurality of exposed regions that expose portions of thesecond polymer layer; etching portions of the second polymer layer thatare exposed by the first masking layer; removing the first maskinglayer; depositing a sacrificial vapor core layer in a pattern on thesecond sacrificial layer; depositing a third polymer layer on thesacrificial vapor core layer; depositing a second masking layer on thethird polymer layer having a plurality of exposed regions that exposeportions of the third polymer layer; etching portions of the thirdpolymer layer that are exposed by the second masking layer; removing thesecond masking layer; removing portions of the sacrificial vapor corelayer using the holes in the third polymer layer; and sealing the holesin the third polymer layer.
 2. The method according to claim 1, furthercomprising depositing the first polymer layer on a substrate.
 3. Themethod according to claim 1, wherein after removing portions of thesacrificial vapor core layer the third polymer layer forms a pluralityof pillars.
 4. The method according to claim 1, wherein the firstpattern comprises a negative of a plurality of pillars.
 5. The methodaccording to claim 1, wherein the second polymer layer comprises a meshlayer.
 6. The method according to claim 1, wherein the masking layercomprises a metal.
 7. The method according to claim 1, wherein themasking layer comprises a plurality of exposed regions.
 8. The methodaccording to claim 1, where the sacrificial vapor core layer may beelectroplated on the second polymer layer.
 9. The method according toclaim 1, where the vapor core layer form a plurality of pillars.
 10. Themethod according to claim 1, wherein removing portions of thesacrificial vapor core layer using the holes in the third polymer layerfurther comprises introducing an introducing an acid through the holesin the third polymer layer.
 11. The method according to claim 1, whereinthe sacrificial vapor core layer has a thickness of less than 200microns.
 12. The method according to claim 1, wherein the second polymerlayer has a thickness of less than 50 microns.
 13. The method accordingto claim 1, wherein the first polymer layer has a thickness of less than50 microns.
 14. The method according to claim 1, wherein removingportions of the sacrificial vapor core layer using the holes in thethird polymer layer further comprises removing portions of the firstsacrificial layer.
 15. The method according to claim 1, whereindepositing a third polymer layer on the sacrificial vapor core layerfurther comprises depositing the third polymer layer into cavities orchannels in the sacrificial vapor core layer.